Differentiation of Magma

Bowen's Reaction SeriesNorman L Bowen was an American geologist who, during the 1920s, carried out a series of experiments to find the temperatures at which minerals in igneous rocks crystallised. He used powdered rock that he heated until it melted and then cooled it down to a specific temperature and observed which minerals crystalissed out. The result was Bowen's Reaction Series, which gives us the order in which minerals crystallise from a magma.Discontinuous SeriesThe left-hand side of a diagram is called the Discontinuous Series. All these minerals are mafic minerals rich in iron and magnesium with silica. In a mafic magma olivine will be the first mafic mineral to form at a high temperature (>1500 degrees). As the temperature lowers, pyroxene, then amphibole and finally biotite will form. If cooling takes place slowly then the early formed, high temperature minerals reaction with the magma to form the next mineral down in the series. For example, olivine reacts with magma to form pyroxene if there is enough silica present. If the magma is cooled quickly when it is erupted from a volcano, then the reaction will not have time to occur and so olivine is preserved. Often the reaction of one mineral with the next will be incomplete and a reaction rim will be seen around the edge of a crystal.Continuous SeriesThe right hand side of the diagram is the Continuous Series, showing the crystallisation of plagioclase feldspar. Plagioclase is calcium rich at the high temperature end and sodium rich at low temperature, with intermediate compositions of 50% calicium and 50% sodium rich at temperatures in between. Plagioclase is continuously reacting with the liquid to form a more Na-rich crystal as the temperature decreases. Sometimes individual crystals of plagioclase will show zoning. The centre of the crystal is Ca-rich plagioclase and towards the edges the crystal is richer in Na-rich plagioclase and poorer in Ca.Knowing that Ca-rich plagioclase is only found in mafic and and ultramafic rocks, while Na-rich plagioclase is found silicic and intermediate rocks, means that they are used in classification of igneous rocks. Low temperature mineralsWhen the two reaction series converge at a low temperature, minerals remain that will not react with the remaining liquid. The final group of minerals to crystallise are the felsic minerals rich in silica. These are are potassium feldspar followed by musocvite mica and finally quartz at a temperature of about 700 degrees.The importance of Bowen's Reaction Series mafic rocks form at high temperatures as they contain olivine, pyroxene and Ca-ric plagioclase silicic rocks form at low temperatures and contain quartz, muscovite, k feldspar and some Na-rich plagioclase and biotite The minerals at the low temperature end of the reaction series are the most stable at the Earth's surface, because they formed in conditions closest to the low temperature surface environment. As a result the most common mineral in sedimentary rocks is quartz the high temperature minerals are most unstable and quickest to to weather at the surface The process of magmatic differentiationMagmatic differentiation is a number of processes that cause a magma to evolve into magmas of different compositions. This leads to different igneous rocks being produced from a single parent magma. A mafic magma may crystallise to form ultramafic, basic, intermediate and even silicic rocks.Fractional CrystallisationAs olivine and pyroxene form at high temperatures, they use iron and magnesium from the magma. The high temperature plagioclase crystals are rich in calcium. The result is that th magma becomes depleted in iron, magnesium and calcium. The remaining liquid becomes enriched in silica, potassium, sodium and water because the early formed minerals are poor in these elements. Over time,. the composition of the magma changes from the original. Gravity settlingGenerally crystals will be denser than the liquid in which they are suspended, so will settle out: early formed minerals with a higher density than the surrounding liquid, such as olivine sink to form a layer at the base of the intrusion. gravity settling removes the crystals from the remaining liquid so they do not react with the remaining magma a layer of dense, early formed minerals at the base of an intrusion or a magma chamber is a cumulate layer Filter PressingDuring crystallisation of magma, there is a point where crystals and liquid exist together as a slushy mess. Due to the weight of the overlying crystals, the liquid gets squeezed out forming a separate layer above. The liquid is depleted in the elements incorporated into the early formed crystals and enriched in the elements which form felsic minerals.

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